Field of the Invention
The present invention relates to a process for the preparation of copolymers of alkyl methacrylates and maleic anhydride having a number average molecular weight of 3000 to 9000 g/mol by solution polymerization in the presence of a radical initiator.
Alkyl (meth)acrylate/maleic anhydride copolymers are known to act as so called flow improvers for hydrocarbon oils such as gas oils, diesel oils, fuel oils, lubricating oils and crude oils. These oils contain a large proportion of long-chain n-paraffins, which crystallize upon cooling and lead to an increased oil viscosity and decreased oil flowability at low temperatures. Alkyl (meth)acrylate/maleic anhydride copolymers can inhibit the crystallization of long-chain n-paraffins and can therefore improve the flow properties of hydrocarbon oils at low temperatures.
Alkyl (meth)acrylate/maleic anhydride copolymers can be prepared by solution polymerization in the presence of a radical initiator. This method requires a solvent, in which both monomers are soluble. Suitable solvents are for example toluene, xylene, methylbenzene, cumene, high boiling point mixtures of aromatics, aliphatic and cyclic aliphatic hydrocarbons, paraffin oils, tetrahydrofuran, and dioxane. Several examples of solution polymerization processes are described in the art.
Description of the Background
CN 1328392 C describes a solution polymerization process, in which an initial solution of maleic anhydride in xylene containing the chain transfer agent n-dodecyl mercaptan is provided at a reaction temperature of 140° C. Two separate solutions containing a radical initiator (di-tert-butyl peroxide) and octadecyl methacrylate, respectively, are then added dropwise to the maleic anhydride solution. Here, addition of octadecyl methacrylate continues even after addition of the initiator has been completed. The resulting copolymer comprises sulfur as a result of the chain transfer agent, which is not desirable for numerous applications which need low-sulfur or even sulfur-free additives.
CN 1302031 C describes a copolymerization process, in which an initial solution of maleic anhydride and tetradecyl methacrylate in toluene is provided at a reaction temperature of 60° C. The initiator (AIBN) is then added to this solution in a single step to start the reaction.
EP 0673990 A1 discloses hydrocarbon oil compositions comprising copolymers made out of at least one C8 to C40 alkyl acrylate units and maleic anhydride units together with at least one additive selected from the group consisting of a homopolymer or copolymer derived from an unsaturated hydrocarbon monomer, an ester and a wax anti-settling additive for improving the cold flow properties of a hydrocarbon oil. All the copolymers disclosed in EP0673990 A1 are made out of maleic anhydride and alkyl acrylate. For the polymerization process, reference is made to the process described in EP 0636637 A2.
EP 0636637 A2 describes a solution polymerization process, in which first a solution of maleic anhydride in toluene is provided at a reaction temperature of 80° C. and is then supplemented with radical initiator (AIBN). After addition of the initiator, a solution of an alkyl acrylate in toluene is slowly added over the course of 100 minutes. In a final step, the obtained alkyl acrylate-maleic anhydride copolymers are purified by dialysis in order to remove the unreacted monomers. All examples provided in EP 0636637 A2 are directed to the polymerization of alkyl acrylate with maleic anhydride.
U.S. Pat. No. 5,178,641 A describes a solution polymerization process, in which a first solution of lauryl acrylate and maleic anhydride in a high boiling point mixture of aromatics (solvesso 150) is provided at a reaction temperature of 100° C. Two separate solutions comprising a radical initiator (tert-butyl-per-2-ethyl hexanoate) in solvesso 150 and lauryl acrylate in solvesso 150, respectively, are then slowly added to the first solution. Here, addition of the initiator continues after addition of lauryl acrylate has been completed. All examples provided in U.S. Pat. No. 5,178,641 A are directed to the polymerization of lauryl acrylate with maleic anhydride.
It is well known in the art that acrylates react differently than methacrylates (e.g. J. Phys. Chem. A 2008, 112, 6772-6782) and it is therefore a challenge to find out adequate reaction conditions in order to improve reaction yields and monomer conversions in a copolymerization reaction with alkyl methacrylates and comonomers, while keeping control over the number average molecular weight.
In the special case where the comonomer is maleic anhydride, the challenge is even higher because the comonomer maleic anhydride is known to have a low reactivity and an excess of maleic anhydride in the reaction solution is usually used as shown in CN 1302031 C, since the known solution polymerization processes suffer from a low conversion of maleic anhydride into the resulting copolymer. However, using an excess of maleic anhydride leads to the problem that polymer chain growth is hindered as soon as all the alkyl methacrylate monomers are consumed, and such processes need an additional purification step to get rid of the unreacted maleic anhydride. This is also the reason why it is difficult to achieve obtaining alkyl methacrylate-maleic anhydride copolymers with high weight average molecular weight, because the excess of maleic anhydride may stop the chain growth.
It was therefore an object of the present invention to develop a process for the preparation of alkyl methacrylate-maleic anhydride copolymers leading to high yields and high monomer conversions for both alkyl methacrylate and maleic anhydride monomers, without any purification step. In particular, it would be desirable to prepare, with high yields and high monomer conversions, sulfur-free copolymers with equimolar contents of alkyl methacrylate and maleic anhydride, which have the required number average molecular weights.
A further challenge was to find reaction conditions that allow controlling the number average molecular weight of the copolymer while maintaining high conversion rate of maleic anhydride into the copolymer, which means a number average molecular weight falling in the range of 3000 to 9000 g/mol together with a low residual or unreacted maleic anhydride in the copolymerization reaction mixture.